Nope, 'vast' means more than very large and its closest synonyms are 'immense' or 'boundless'. I would consider a vast majority to require at LEAST an order of magnitude difference, so 90%+ as a bare minimum and I would probably not use it myself until more like 95% and I would still consider that rather hyperbolic up until 99%. You could call 66% a strong majority or a clear majority, but 'vast' is far too strong in my opinion.